The present invention relates to a method for manufacturing an infrared detecting device.
There has been known an infrared detector using silicon diodes. Each of the silicon diodes has a temperature characteristic that its output voltage changes when the temperature changes under constant current environment, and detects infrared rays or radiation using this property. In order to enhance the sensitivity of a conventional infrared detector. comprised of silicon diodes, the infrared detector is configured in which as shown in, for example, a patent document 1 (Japanese Patent Application No. 4011851), the plural silicon diodes are connected in series through connecting wirings and elements or devices (hereinafter called infrared detecting devices) equipped with the silicon diodes connected in series are disposed in plural form. In the devices shown in the patent document 1, the series-connected silicon diodes are disposed with being folded back to reduce the entire area of devices.
It is general that the shape of each infrared. detecting device has heretofore been assumed as square. The merit of the square shape resides in that the degree of difficulty in processing of the two-way (vertical/horizontal) photolithography technology is low. FIG. 9(A) is a view showing a layout example where square-shaped infrared detecting devices are disposed in linked form. In FIG. 9(A), reference numerals 100 indicate infrared detecting devices. Each of the infrared detecting devices 100 includes a silicon diode section 102 (typically represented as rectangular herein) in which silicon diodes are connected in series, beam portions 104 used as supporters or support bodies for supporting the silicon diode section 102, metal wirings, although not shown in the figure, for connecting the silicon diodes of each silicon diode section 102, etc. The infrared detecting devices 100 are respectively separated by a square-shaped isolation portion 108.
FIG. 9(B) is a view typically showing one example of a sectional view of each infrared detecting device 100. The infrared detecting device 100 is provided with an infrared absorber 112 above its corresponding silicon diode section 102 including plural silicon diodes 120 connected in series, and configured in such a manner that a rise in temperature produced due to the absorption of infrared radiation by the infrared absorber 112 is transmitted to the silicon diode section 102 (incidentally, the infrared absorber 112 is not illustrated in FIG. 9(A)). As shown in FIG. 9(B), a hollow portion 116 is formed in a silicon substrate 110 lying below the silicon diode section 102. The silicon substrate 110 lying below the silicon diode section 102 is hollowed out in this way thereby to enhance the rate of rise in temperature of a sensing portion. Incidentally, an infrared detector in which a hollow portion is formed in a substrate lying below a sensor section has been included even in a patent document 2 (Japanese Unexamined Patent Publication No. Hei 9(1997)-126895).
The hollow portion 116 is formed by providing etching holes 106 (refer to FIG. 9(A)) in each infrared detecting device 100 and supplying XeF2 gas or the like through the etching holes 106. The etching holes 106 are not illustrated in FIG. 9(B). Since this etching approximates to isotropy, the hollow portion 116 is formed so as to spread in circular form. From this point of view, it is desired that as a method for laying out the etching holes 106, the etching holes 106 are placed or laid out in the center of each infrared detecting device 100 or the etching holes 106 are disposed so as to have symmetry. Since the silicon diodes are placed in the center of each infrared detecting device in FIG. 9(A), the layout of the etching holes having symmetry has been adopted.
When the shape of each device is square (refer also to the patent document 1), the shortest distance between one side of the isolation portion 108 and the etching hole 106 and the distance between the corner of the isolation portion 108 and the etching hole 106 are much different from each other as shown in FIG. 9(C). Therefore, a difference occurs between the timing provided to allow an etchant to reach the one side and the timing provided to allow the etchant to reach the corner.
Thus, there is a possibility that since the etchant reaches the corner after the etchant has reached the side of the isolation portion 108 upon etching processing, the side is over-etched, so that the etchant will reach beyond the isolation portion 108 during this period (refer to the arrow shown in FIG. 9(B)). When the etchant reaches beyond the isolation portion 108, the hollow portions 116 of the adjoining infrared detecting devices 100 are coupled. Therefore, the structure of the infrared detecting device becomes instable physically. The structure is considered also to have a possibility of being lifted off in some cases.
Incidentally, when the etching processing is performed in alignment with the side of the isolation portion 108, there is the potential that the etchant will not reach the corner sufficiently this time. In this case, hollowing cannot be carried out sufficiently.
As means for solving such a problem, there are cited a method for forming the isolation portion 108 deep, and a method for devising the layout of each etching hole 106 to reduce the degree of overetch.
When, however, the former method is adopted, a dedicated deep etcher (DRIE) is required separately. When the latter method is adopted, there is cited a method for opening a large number of etching holes. Since, however, the etching holes 106 cannot be laid out in the areas for forming the silicon diodes 120 and the beam portions 104, there also occurs a case in which the etching holes 106 cannot be placed in their corresponding required locations.
Incidentally, although the, infrared detector in which the hollow portion is formed in the substrate lying below the sensor section, has been included even in the patent document 2, the infrared detector described in this document is not formed with the isolation portion because it is of one formed by a single device or element. Thus, this does not take into consideration the problem that when a plurality of devices are disposed continuously, overetching is done beyond the isolation portion.